In wavelength division multiplexed (WDM) optical communications networks operating at data rates of up to 40 Gb/s, 100 Gb/s or higher, each channel must be launched at a desired optical power level in order to obtain a desired signal quality. This is particularly important for optical communications systems operating at a high spectral efficiency (number of bits per second per unit bandwidth, bit/s/Hz). Channel power setting is made by a Channel Monitoring unit (CMU), as set out in ITU-T Recommendation G.697, that monitors an aggregate WDM data traffic signal.
CMUs typically provide channel power measurements by estimating the spectral power density of an aggregate WDM data traffic signal using a tunable filter and a photodiode. The power measurements are given referred to an equivalent optical spectrum analyzer (OSA) resolution. These channel power measurements are systematically inaccurate due to the spectral resolution of the CMU, and CMU manufacturers provide correction factors for the measurements for a range of data signal rates and modulation formats. Many commercial CMUs cannot meet the ITU-T Recommendation G.697 requirements for channel power measurement accuracy for traffic signals with high data rate and high SE.
In another approach, the WDM traffic signal is demultiplexed and the optical power of each channel is measured using a photodiode. The disadvantage of this approach is that the information about the carrier frequency and the optical signal to noise ratio (OSNR) is lost and the demultiplexer can imperfectly select a channel and insulate the other channels. In U.S. Pat. No. 6,871,020, directed to the minimization of channel crosstalk, a series of optical splitter and wavelength selective filters (fibre Bragg gratings) is used to separate out the WDM channels, for power measurement using photodiodes. This approach would be difficult to scale to the number of channels (80, 96 and more) required in current optical communications networks.